1. Field of the Invention
This invention generally relates to coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a thermal barrier coating (TBC) having a columnar microstructure, an interior region formed of a ceramic material with lower thermal conductivity than yttria-stabilized zirconia (YSZ), and an outer surface region that is more erosion-resistant than the interior region of the TBC.
2. Description of the Related Art
Components within the hot gas path of gas turbine engines are often protected by a thermal barrier coating (TBC). TBC's are typically formed of ceramic materials deposited by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques. Spraying techniques deposit TBC material in the form of molten splats, resulting in a TBC characterized by a degree of inhomogeneity and porosity. TBC's employed in the highest temperature regions of gas turbine engines are most often deposited by PVD, particularly electron-beam PVD (EBPVD), which yields a strain-tolerant columnar grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., laser melting, etc.).
Various ceramic materials have been proposed as TBC's, the most widely used being zirconia (ZrO2) partially or fully stabilized by yttria (Y2O3), magnesia (MgO), or ceria (CeO2) to yield a tetragonal microstructure that resists phase changes. Other stabilizers have been proposed for zirconia, including hafnia (HfO2) (U.S. Pat. No. 5,643,474 to Sangeeta), gadolinia (Gd2O3) (U.S. Pat. Nos. 6,177,200 and 6,284,323 to Maloney), and dysprosia (Dy2O3), erbia (Er2O3), neodymia (Nd2O3), samarium oxide (Sm2O3), and ytterbia (Yb2O3) (copending U.S. patent application Ser. No. 10/064,939). Yttria-stabilized zirconia (YSZ) has been the most widely used TBC material due at least in part to its high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and PVD techniques.
TBC materials that have lower thermal conductivities than YSZ offer a variety of advantages, including the ability to operate a gas turbine engine at higher temperatures, increased part durability, reduced parasitic cooling losses, and reduced part weight if a thinner TBC can be used. Commonly-assigned U.S. Pat. No. 6,586,115 to Rigney et al. discloses a YSZ TBC alloyed to contain an additional oxide that lowers the thermal conductivity of the base YSZ composition. These additional oxides include alkaline-earth metal oxides (magnesia, calcia (CaO), strontia (SrO) and barium oxide (BaO)), rare-earth metal oxides (ceria, gadolinia, neodymia, dysprosia and lanthana (La2O3)), and/or such metal oxides as nickel oxide (NiO), ferric oxide (Fe2O3), cobaltous oxide (CoO), and scandium oxide (Sc2O3). According to Rigney et al., when present in sufficient amounts these oxides are able to significantly reduce the thermal conductivity of YSZ by increasing crystallographic defects and/or lattice strains. Other ternary YSZ coating systems that have been proposed include YSZ+hafnia (commonly-assigned U.S. Pat. No. 6,352,788 to Bruce) and YSZ+niobia (Nb2O3) or titania (TiO2) (commonly-assigned co-pending U.S. patent application Ser. No. 10/063,810 to Bruce et al.).
In addition to low thermal conductivities, TBC's on gas turbine engine components are required to withstand damage from impact by hard particles of varying sizes that are generated upstream in the engine or enter the high velocity gas stream through the air intake of a gas turbine engine. The result of impingement can be erosive wear (generally from smaller particles) or impact spallation from larger particles. Many of the oxides noted by Rigney et al. as able to reduce the thermal conductivity of YSZ have the disadvantage of also reducing erosion resistance. Above-noted U.S. Pat. No. 6,352,788 to Bruce teaches that YSZ containing about one up to less than six weight percent yttria in combination with magnesia and/or hafnia exhibits improved impact resistance. In addition, commonly-assigned U.S. patent application Ser. No. 10/063,962 to Bruce shows that small additions of lanthana, neodymia and/or tantala to zirconia partially stabilized by about four weight percent yttria (4% YSZ) can improve the impact and erosion resistance of 4% YSZ.
It would be desirable if improved TBC's were available that exhibited both lower thermal conductivities and improved erosion resistance.